In the synthesis of organohalosilanes by the so-called Direct Process (Rochow, U.S. Pat. Nos.: 2,380,995, 2,380,996 and 2,380,997 issued Aug. 7, 1945), i.e., by receiving powdered silicon and organohalides in the presence of a metallic catalyst especially copper, it is well known that the addition of zinc as a promoter improves both reaction rate and selectivity of organohalosilanes as shown, for instance, by W. S. Ward III et al., U.S. Pat. No. 4,500,724.
The properties desired in a good catalyst for the Direct Process include high selectivity, i.e., a high percentage of dimethyldichlorosilane(D) in the crude product, high activity, i.e., a high rate of formation of the desired product(s); and high silicon conversion, i.e., utilization or conversion of the silicon powder to a high degree without excessive loss of selectivity and activity. See also "Surface Analysis of Methylchlorosilane Formation Catalysts", Frank et al, Journal of Catalysis 95, 396-405 (1985).
Since Rochow's description of the Direct Process in 1945, many modifications and improvements have been described. The most commonly used catalyst is based on copper. Metallic copper, copper chloride, and oxide type or partially oxidized copper have been described in the patent literature and are successfully used in commercial reactors. See U.S. Pats. Nos. 4,504,597 dated Mar. 12, 1985, and 4,520,130 dated May 28, 1985.
One of the most widely used promoters, described by Gilliam in U.S. Pat. No. 2,464,033 is zinc or zinc halides and their mixtures. Zinc compounds such as halides of zinc and zinc oxide have been suggested as effective catalyst promoters equivalent to metallic zinc (U.S. Pat. No. 4,500,724; Ward et al).
See also Hurd, U.S. Pat. No. 2,427,605 dated Sept. 16, 1947; Bluestein, U.S. Pat. No. 2,887,502 dated May 19, 1959; Takami et al U.S. Pat. No. 2,903,473 dated Sept. 8, 1959; Rossmy, U.S. Pat. No. 3,069,452 dated Dec. 18, 1962; Turetskaya et al, U.S. Pat. No. 3,555,064 dated Jan. 12, 1971; Marko et al, U.S. Pat. No. 4,578,494 dated Mar. 25, 1986; and Prudhomme, U.S. Pat. No. 4,645,851 dated Feb. 24, 1987. The patent to Reed U.S. Pat. No. 2,389,931 dated Nov. 27, 1945 discloses improved results obtained using a fluidized bed technique which process may also be used herein.
In the manufacture and use of promoter-containing catalysts certain combinations of catalysts, promoters, and activators can give rise to highly exothermic mixtures, particularly if present within certain concentration ranges as may be the case during manufacture and/or blending of the various components of a complete catalyst or of the contact mass itself. Examples of such exothermic mixtures that represent potential explosion hazards are based on the following chemical reactions: EQU Zn'Cu.sub.2 O.fwdarw.2Cu+ZnO+Heat EQU Zn+CuO.fwdarw.Cu+ZnO+Heat EQU Zn+2CuCl.fwdarw.2Cu+ZnCl.sub.2 +Heat
Thus, if zinc powder is blended, for instance, with copper oxide or copper chloride prior to its use in the silane reactor, special precautions must be taken. In working with zinc promoters that do not exhibit these hazards, such as for instance, zinc formate, zinc acetate, and zinc carbonate, the inventors have now discovered to their surprise that the use of certain of these zinc compounds, as the zinc based promoter, is superior to zinc powder or zinc halide especially zinc chloride with respect to one or more of the desirable zinc chloride with respect to one or more of the desirable characteristics mentioned above. Furthermore, not only do these zinc compounds form non-explosive mixtures with copper oxide, they also are, unlike zinc chloride, non-hygroscopic or less hygroscopic and thus do not impair the flow properties of the catalyst or the catalyst-silicon contact mixture to the same degree. Thus, with one of these zinc compounds as the principal promoter, one can now prepare and use fully promoted and activated catalysts exhibiting superior performance and free from the hazards of explosiveness and flow deterioration.
In our experiments, we have found the use of certain zinc compounds to give either better selectivity, higher activity, or a superior degree of silicon conversion than that obtained with metallic zinc, zinc oxide, or zinc chloride.